1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for transmitting and receiving a cell identification code that identifies a base station (BS) in an orthogonal frequency division multiple access (OFDMA) mobile communication system.
2. Description of the Related Art
In a typical code division multiple access (CDMA) mobile communication system, for example, an IS-95 CDMA system, a mobile station (MS) performs an initial cell search at a power-on to acquire pseudo noise (PN) code timing. A BS sends a PN code to all MSs within its coverage area on a forward pilot channel. The forward pilot channel delivers unmodulated data, which is spread with a PN code, and the MSs acquire the PN code timing from the forward pilot channel.
In a cell search in the IS-95 CDMA system, all BSs are synchronized to one another with the aid of global positioning system (GPS) satellites. The BSs use the same PN code at different offsets, for their identification. At the power-on, an MS acquires the PN code timing of its serving BS from a forward pilot channel and performs an initial cell search. During the initial cell search, the MS generally performs correlations on a received signal, shifting a search window of a size equal to the length of the PN code. The cell search lasts until a maximum correlation value is detected and the serving cell is identified by acquiring a PN code phase having the maximum correlation value.
This IS-95 CDMA system has evolved to 3rd generation (3G) mobile communication systems including universal mobile telecommunication system (UMTS). Based on CDMA, the UMTS system is asynchronous between Node Bs. A user equipment (UE), which is equivalent to an MS in the IS-95 CDMA system in concept, also performs an initial cell search. While the IS-95 CDMA system and the UMTS system commonly work on CDMA, the former is synchronous and the latter is asynchronous. Additionally, they differ in the initial cell search.
In a cell search in the UMTS system, each Node B is assigned a cell-specific code, for identification. Given 512 cells each having one Node B, 512 Node Bs exist in the UMTS system, which are identified by their specific cell identification codes. A UE must search the 512 respective Node Bs for its serving Node B. Because the cell search is about checking, one by one, the phases of the cell identification codes of the 512 Node Bs, it takes a lot of time to search for the serving Node B. Consequently, it is inefficient to use this general cell search algorithm in which the highest correlation energy value is detected. Therefore, the UMTS system utilizes a multi-stage cell search algorithm.
In the above example, the 512 Node Bs are grouped into a predetermined number of groups, for example, 64 groups, each having 8 Node Bs. Different group identification codes are assigned to the 64 respective groups and the 8 Node Bs of each group are identified by spreading codes (or scrambling codes) used for their common pilot channels (CPICHs). Therefore, the UE first acquires a Node B group and then identifies a Node B by correlating a received CPICH with the scrambling codes of the Node Bs in the Node B group.
For the UMTS system, site selection diversity transmission (SSDT) has been proposed. Herein, the term “site” is interchangeable with “BS” and “cell” in its meaning.
The SSDT is a macro-diversity method to be used in a soft handover mode, activated by the system. In an SSDT procedure, the UE selects a “primary cell” among the cells of an active set, all other cells being classed “non-primary”. A main objective of the SSDT is to transmit on the downlink from the best cell (hereinafter, referred to as the primary cell), thus reducing the interference caused by multiple transmissions in a soft handover mode.
Each active cell that transmits at or above a predetermined power level is assigned a temporary identification. The UE periodically measures the reception levels of common pilots transmitted by the active cells and selects a cell with the highest pilot power as a primary cell. The non-primary cells then switch off their transmit power by the UE. The temporary identification of the primary cell is used as a site selection signal being a Hadamard code-based binary sequence of a predetermined bit length.
As described above, many binary codes are used for cell identification, such as PN codes and Hadamard codes. Generally, the performance of a cell identification code depends on a maximum auto-correlation, a maximum cross-correlation, or a minimum Hamming distance. Accordingly, a cell identification code must be designed that maximizes the maximum auto-correlation, minimizes the maximum cross-correlation, or maximizes the minimum Hamming distance.
FIG. 1 is a schematic block diagram of an apparatus for transmitting a cell identification code in a Node B. Referring to FIG. 1, a cell identification code generator 105 generates a scrambling code or a spreading code as a cell identification code in a predetermined method. A scrambler/spreader 103 scrambles/spreads input data with the scrambling/spreading code. The scrambling/spreading code itself is transmitted over a stream of all Is or over data through scrambling/spreading.
As described above, a cell identification code is a PN code or a Hadamard code and acquisition of the cell identification code of a serving cell includes searching for a code having a maximum auto-correlation value. In the UMTS system, a UE correlates a received CPICH signal with a plurality of PN codes (or scrambling codes) and selects correlation values equal to or greater than a threshold, to thereby acquire the cell identification code of a serving Node B. For a fast cell search, the UE may be provided with a plurality of correlators. For example, the UE is equipped with N correlators 210 to 270 in the case illustrated in FIG. 2. All correlators operate in the same manner and thus the operation of the first correlator 210 will be described by way of example.
FIG. 2 is a schematic block diagram of an apparatus for receiving a cell identification code in a UE. Referring to FIG. 2, upon receipt of a CPICH signal, its I- and Q-channel components RX_I and RX_Q are provided to a decimator 211 of the first correlator 210. The decimator 211 selects predetermined samples from the I- and Q-channel components RX_I and RX_Q at a timing for cell search, and outputs one sample per chip to a descrambler 213. The descrambler 213 descrambles the signal received from the decimator 211 with a scrambling code & symbol pattern generated from a scrambling code & symbol pattern generator 215 and provides the descrambled I-channel component to a first primary accumulator 217 and the descrambled Q-channel component to a second primary accumulator 219. The primary accumulators 217 and 219 accumulate the I- and Q-channel components, respectively, a predetermined number of times. An energy calculator 221 squares the I-channel and Q-channel accumulations and sums the squares, thereby calculating a correlation energy. A secondary accumulator 223 accumulates the correlation energy a predetermined number of times.
All other correlators calculate correlation energies in the same manner as the first correlator 210. A maximum value detector 225 compares the correlation energies received from the first to Nth correlators 210 to 270 each with a threshold. If there is at least one correlation energy at or above the threshold, the maximum value detector 225 determines that it has succeeded in the cell search.
Mobile communication technology has recently evolved to 4th generation (4G) mobile communications. More specifically, the OFDMA system, which attracts more interest as a promising 4G mobile communication system, uses pilot subcarriers for cell identification, as compared to the systems described above. Data subcarriers and pilot subcarriers are spread with orthogonal codes and different orthogonal codes are used in spreading pilot subcarriers for different Node Bs, for Node B identification. The number of identifiable Node Bs is limited by the spreading factor (SF) of the orthogonal codes used. To identify more Node Bs, an entire time-frequency area assigned to each Node B is divided into smaller time-frequency areas each being assigned a different spreading code for pilot subcarriers, and a Node B is identified by a sequence of orthogonal codes used for pilot subcarriers. This method advantageously enables even sector identification as well as Node B identification.
An orthogonal code used for spreading can be detected through correlation, after despreading at a receiver. Because pilot subcarriers are transmitted with high power relative to data subcarriers, the receiver sets an orthogonal code having the highest correlation value as one for a pilot subcarrier.
As described above, many communication systems use an orthogonal code as a cell identification code. However, the orthogonal code is susceptible to errors over a channel, thereby degrading cell identification performance. In this context, the receiver must use a cell identification code that maximizes an auto-correlation value in order to improve the cell identification performance. Accordingly, a cell identification code may be transmitted at a high power level, but the increase of transmit power may interfere with signals from neighboring Node Bs and increase hardware complexity and hardware cost in the transmitter and the receiver.